In a UMTS (Universal Mobile Telecommunications System) network, for the purposes of improving frequency usage efficiency and improving the data rates, system features which are based on W-CDMA (Wideband Code Division Multiple Access) are maximized by adopting HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access). For this UMTS network, for the purposes of further increasing high-speed data rates, providing low delay and so on, long-term evolution (LTE) has been under study.
In the third-generation mobile communication system, it is possible to achieve a transmission rate of maximum approximately 2 Mbps on the downlink by using a fixed band of approximately 5 MHz. Meanwhile, in a system of the LTE scheme, it is possible to achieve a transmission rate of about maximum 300 Mbps on the downlink and about 75 Mbps on the uplink by using a variable band which ranges from 1.4 MHz to 20 MHz. Furthermore, in the UMTS network, for the purpose of achieving further broadbandization and higher speed, successor systems of LTE have been under study (for example, LTE-Advanced (LTE-A)). For example, in LTE-A, there is a plan to expand the maximum system band for LTE specifications, which is 20 MHz, to approximately 100 MHz.
Also, in a system of the LTE scheme, a MIMO (Multi Input Multi Output) system is proposed (see, for example, non-patent literature 1), as a radio communication technique to transmit and receive data by a plurality of antennas and improve the data rate (frequency usage efficiency). In the MIMO system, a plurality of transmission/reception antennas are provided in the transmitter/receiver, so that different transmission information sequences are transmitted from different transmission antennas, at the same time. On the other hand, on the receiver side, taking advantage of the fact that fading variation is produced between transmission/reception antennas, by separating and detecting the information sequences that have been transmitted at the same time, it is possible to increase the data rate (frequency usage efficiency).
In a system of the LTE scheme, single-user MIMO (SU-MIMO), in which all the transmission information sequences that are transmitted from different transmission antennas are directed to the same user, and multiple-user MIMO (MU-MIMO), in which transmission information sequences that are transmitted from different transmission antennas are directed to different users, are defined. In the LTE system of Release 8 (Rel. 8), SU-MIMO and MU-MIMO are defined with different transmission modes, and are both defined with different downlink control channel (DCI (Downlink Control Information)) formats. For example, SU-MIMO is set in transmission mode 3 (open loop) and transmission mode 4 (closed loop), and MU-MIMO is set in transmission mode 5. The number of bits in the DCI format and information to be contained in the DCI format vary between these transmission modes. It is possible to switch between these transmission modes semi-statically.
Each transmission mode can be set in two DCI formats depending on usage. For example, in transmission mode 5, which corresponds to MU-MIMO, DCI formats 1D and 1A are defined. In the same transmission mode, it is possible to switch the DCI format dynamically. On the mobile station apparatus (UE) side, blind detection of the DCI format, which is selected in the base station apparatus, is performed. Consequently, as the number of DCI formats increases, the number of searches for blind detection increases. However, in the LTE system of Rel. 8, two DCI formats are allowed to be included for one transmission mode.
On the other hand, in the LTE system of Release 9 (Rel. 9), SU-MIMO and MU-MIMO, which use user-specific reference signals for demodulation (UE-specific RSs), are defined with the same DCI format (DCI format 2B). Consequently, in the LTE system of Rel. 9, SU-MIMO, in which the maximum number of transmitting streams is two, and MU-MIMO, in which the maximum number of users to multiplex is two, can be switched dynamically. Also, given that the transmission mode to include DCI format 2B includes, besides this, DCI format 1A alone, it is possible to maintain the number of searches for blind detection on the mobile station apparatus side the same number of times as in Rel. 8.